Electrographic position location apparatus and method

ABSTRACT

An apparatus for use in an electrographic position sensing system comprises an antenna system and a signal strength detector. In one embodiment, the antenna system comprises two antennas. The detector measures the signal strength from each antenna. A microprocessor contains an algorithm to calculate the position of the detector near the antennas.

This application is based on and claims the priority of provisionalpatent application Ser. No. 60/200,722, filed on Apr. 27, 2000,incorporated herein by reference, and provisional patent applicationSer. No. 60/200,960, filed on May 1, 2000, Ser. No. (to be assigned),incorporated herein by reference.

BACKGROUND OF THE INVENTION

There are a variety of games, toys, and interactive learning devices inwhich a stylus is used to point to a region on a surface in order toinput data or questions. There are several technologies to determine theposition of a stylus on a sensing surface. One approach is to embed anarray of pressure sensitive switches in the sensing surface, such asmembrane switches. However, conventional membrane switches have limitedresolution. Another approach consists of arrays of capacitive orinductive elements whose impedance is altered by bringing the stylusinto contact with the surface. However, a disadvantage of this approachis that a large number of pixel elements are required to achieve a highresolution. Moreover, since capacitive and inductive effects aretypically small, the stylus must be brought into close proximity to thepixel in order to obtain a strong position signal.

In many applications it is desirable to be able to determine theposition of a stylus disposed a short distance away (e.g., 1 mm to 2 cm)from an electrically active surface. In many consumer products it isdesirable to protect electrically active elements with a protectivelayer of plastic which is thick enough to provide both mechanical andelectrical insulation. The insulating material, such as a layer ofplastic, may also be patterned with numbers, indicia, symbols, anddrawings which facilitate the user inputting data by pointing to anumber, indicia, symbol, or drawing disposed on the surface of theplastic. Other applications include systems in which the number,indicia, symbol, or drawing is disposed on a top (open) page of abooklet. The position of a pointer disposed on the open page of thebooklet may be sensed even though the pointer is separated from theactive surface by the thickness of the booklet.

An electrographic sensor unit and method based upon a geometricalgorithm that is described in U.S. Pat. No. 5,686,705 “Surface PositionLocation System and Method” and U.S. Pat. No. 5,877,458 “SurfacePosition Location System And Method,” which is assigned to the assigneeof the present invention. According to the teachings of U.S. Pat. Nos.5,877,458 and 5,686,705 the position of a stylus is determined bycalculating the intersection point of equipotential lines based upon themeasured signal strength received by the stylus. The contents of U.S.Pat. Nos. 5,686,705 and 5,877,458 are hereby incorporated by referencein the present application.

FIGS. 1-4 show the general principals of the geometric location methodof U.S. Pat. Nos. 5,686,705 and 5,877,458. FIG. 1 is a simplifiedgeometry illustrating the basic principles of operation. As shown inFIG. 1, a two or three dimensional conductive surface has a selectedresistivity. In the embodiment of FIG. 1, three electrical contacts 12,14, and 16 are connected to conductors 24, 26, and 28, respectively, toa processor 30. Also connected to processor 30 is conductor 18 withstylus 20 having a tip 22 for the user to indicate a position on thesurface 10 that is of interest to the user. As shown in FIG. 2, when auser selects a point, P, on resistive surface 10, a series of fieldpotential measurements are performed to calculate the position of thestylus. A DC offset value is determined with no radio-frequency (rf)signals applied to any of the contacts 12, 14, and 16. A secondmeasurement is made by applying an equal amplitude rf signal to allthree contacts 12, 14, and 16, and processor 30 measures the full-scalesignal value via stylus 20. A third measurement is made by applying anrf signal to one of the contacts, such as contact 12, with a secondcontact grounded, such as contact 14. The signal measurement made bystylus 20 will lie somewhere along an equipotential line between thosetwo contacts (i.e., line X in FIG. 2). A fourth measurement is made byapplying the signal to, and grounding a different pair of contacts, say12 and 16, and the signal measurement made with stylus 20 which will besomewhere along an equipotential line between those two contacts (i.e.,line Y in FIG. 2) with the position of the stylus 20 being theintersection of lines X and Y. For the. purposes of illustration, linesX and Y are shown as straight lines. More generally the actual positionof the stylus on the surface can be determined using mathematically orempirically determined models of the signal level gradients for thesurface material with curved equipotential lines.

FIG. 3 illustrates an embodiment of an electrographic sensor system ofU.S. Pat. No. 5,877,458 having a rectangular shaped piece of conductivematerial as sheet 100. Afixed near the edge of sheet 100, and makingelectrical contact thereto, are contacts 102, 104, and 106. Connectedbetween contacts 102, 104, and 106 on sheet 100 and contacts 126, 128,and 130 of signal generator 122, respectively, are electricallyconductive leads 108, 110, and 112. Signal generator 122 includes an rfgenerator 124, amplifier 134, and switches 132 and 136 to determinewhich signals are fed to contacts 126, 128, and 130. The position ofswitches 132 and 136 is controlled via cables 138 and 140, respectively,from microprocessor 142 to select which contacts 102, 104, and 106receive an normal or inverted rf signal.

Stylus 116 contains a receiving antenna and is coupled to signalmeasurement stage 120 via cable 118. The signal is demodulated andturned into a digital signal via demodulator 144 and analog to digitalconverter (ADC) 146. ADC 146 presents the digitized signal tomicroprocessor 142. Microprocessor 142 includes RAM 145, ROM 147, aclock 148 to contain information related to the position that has beenpre-stored along with an audio card 150 and speaker 154 or monitor 152to output information on the selected area.

When an rf signal is coupled to one or more of the contacts 102, 104,and 106 the signal radiates through the conductive material of sheet100. Between a given set of energized contacts, such as contacts 102 and104, a signal level equipotential map 114A exists because of thedistributed resistance in the conductive material of sheet 100. Thesignal level equipotential map includes the shape and values of theequipotential lines and may be stored in the memory of themicroprocessor or the ROM 147. The shape of the these equipotentiallines may, in principal, be calculated by finding the unique solution ofmathematical equations or may be determined empirically. Additionally,there will be a signal equipotential map for other sets of energizedcontacts, such as equipotential map 114B for energized contacts 102 and106. The measurement of the signal strength received at the stylus for aparticular set of energized contacts may be used to calculate whichequipotential line the stylus lies on. The measurement of two sets ofenergized contacts with substantially orthogonal equipotential linespermits the position of the stylus to be calculated, as indicated bypoint P of FIG. 3.

FIG. 4 has similar elements as for FIG. 3 as applied to a globe havingtwo hemispherical conducting surfaces 701 and 702. Insulating mapsurfaces 601 and 602, containing details of world geography, are shapedto house hemispherical surfaces 701 and 702. Hemisphere 701 has contacts710, 711, and 712. Hemisphere 702 has contacts 740, 741, and 742.Switches 770, 771, 772, and 773 along with cables 730, 750 and leads760, 761 of signal generator 722 are configured so that each hemisphere701 and 701 is driven in a manner similar to that of sheet 100. However,the equipotential maps for a hemispherical surface energized by twocontacts, such as contacts 710 and 711, is typically more complex thanfor sheet 100 because of the spherical geometry. Additionally themathematical algorithms must be calculated in spherical coordinates.

The electrographic apparatus and method of U.S. Pat. Nos. 5,686,705 and5,877,458 has many applications, such as interactive globes. Oneadvantage of the electrographic sensor technology of U.S. Pat. Nos.5,686,705 and 5,877,458 is that the mechanical construction iscomparatively simple and inexpensive. The conductive surface 100 or 701,702 may be formed using a variety of deposited or coated materials. Theposition resolution is superior to many competing technologies, makingit desirable for a variety of educational toys. For many applicationsthe position of the stylus may be calculated to within severalmillimeters, making the electrographic apparatus of U.S. Pat. Nos.5,686,705 and 5,877,458 useful in a variety of interactive games, suchas the EXPLORER GLOBE™, sold by LeapFrog Toys of Emeryville, Calif.However, the inventors of the present application have recognizedseveral drawbacks to the electrographic apparatus of U.S. Pat. Nos.5,686,705 and 5,877,458. One drawback is that significant electronicmemory and computing time is required to perform the mathematicalcalculations. In order to convert measured signal strengths intoposition data an equipotential map or equation is useful. Theequipotential lines between energized point contacts on solidtwo-dimensional surfaces, or surfaces having uniform resistivity, havenon-linear, non-parallel and curved contours which lead to complicatedalgorithms for position determination. The complicated algorithms, inturn, result in relatively expensive and slow electronics. Additionally,in some topologies, such as that of hemisphere 701, the curved geometryfurther complicates the calculation of the shape of the equipotentiallines. Consequently, significant memory and computing time is requiredto perform each position calculation.

Another drawback with the electrographic location position sensingsystem of U.S. Pat. Nos. 5,686,705 and 5,877,458 is that the positionsensing resolution tends to degrade towards the edges and corners of theactive surface. The position sensing method of U.S. Pat. Nos. 5,686,705and 5,877,458 is based upon calculating the intersection ofequipotential lines from different pairs of energized contacts. However,the equipotential lines tend to be parallel near the edges and corner ofcommon surface shapes. As is well known, it is difficult to obtainaccurate measurements of the position of a point based upon theintersection of two nearly parallel lines because a small empiricalvariation in measured data produces large variations in the calculatedintersection point. Consequently, position resolution will tend be poorin regions of surface 100 or 701 where the equipotential lines ofdifferent pairs of energized contacts are nearly parallel. Experimentsby the inventors with hemispheres 701, 702 similar to those shown inFIG. 4 indicate that there is a region around the rim of a hemisphere701, 702 with greatly reduced position resolution capability, which theinventors attribute to nearly parallel equipotential lines near the edgeof a hemisphere 701, 702. This makes it difficult, for example, todesign an interactive learning globe in which the user can point tosmall countries located close to the equator (e.g., Equatorial Guinea)to obtain information on the country. Similarly a rectangular conductivesurface, such as surface 100, there also tends to be a region of reducedresolution near the edges of surface 100, making it difficult, forexample, to identify small countries or regions located on the edge of aplanar map. Further the equipotential lines for a planar surface ofuniform resistance are curved and generally less orthogonal andtherefore more complex than is desirable as illustrated in FIGS. 5A and5B. This results in complex and slow mathematical algorithms.

Common techniques to form a continuous resistive coating on a surface100 or 701 result in significant spatial variations in thickness and/orresistivity. In a single fabrication lot there can be substantialvariations in the electrical resistance of each surface. This variationin resistivity across the sensing surface can significantly effect thecontours of the equipotential lines. Therefore, it is necessary tocompensate for those effects with a two-dimensional algorithm that leadsto complex and time-consuming manufacturing processes. Consequently, alarge number of data points are required to accurately map theequipotential lines. Additionally, a large amount of data must typicallybe stored in an equipotential map. This increases product cost.

An electrographic position sensing system using a similar calculation toU.S. Pat. Nos. 5,686,705 and 5,877,458 is desirable because of thepotential for high accuracy, low manufacturing cost, and comparativelysimple construction. However, previously known electrographic positionsensing systems suffer from the problems of reduced resolution alongedge regions because of the substantially parallel equipotential linesdisposed along edge regions, the requirement of significantcomputational memory and computing time to calculate a position basedupon complicated equipotential contours, and the need to performcomplicated calibration procedures to map the equipotential lines.

What is desired is an improved electrographic apparatus and methodproviding improved control of the equipotential signal contours.

SUMMARY OF THE INVENTION

The present invention is generally directed towards an electrographicposition sensing system, including antenna apparatus to generateelectropotential gradients in an electrographic position detectingsystem, a method of manufacturing the antenna apparatus, and the use ofthe antenna apparatus in an electrographic position system.

The novel and inventive antenna apparatus described herein can radiate atwo dimensional electric field potential that can be properly describedby set of vertical or horizontal equipotential field lines, each linehaving a different potential value associated with it. The magnitude andgradient of the radiated field lines is easily designed into the antennaapparatus. Because the generated field potential is easily calculated asa function of the antenna design, it can be used to locate which line areceiving antenna lies on. As a receiving antenna is moved from oneequipotential line to another, it will pick up the field strength of theline over which it is located. If the receiving antenna is placed in asingle location over the radiating (or transmitting) antenna, themagnitude of the potential the antenna senses will reveal on whichequipotential line the receiving antenna is located.

If a user desires to know more about the position of the receivingantenna than the equipotential line on which it resides, a secondtransmitting antenna can be used. The second transmitting antenna can beoriented a 90° to the first transmitting antenna. In this configuration,a user can cause first one antenna to broadcast a set of equipotentiallines, and having located the receiving antenna on a line, the user canthen cause the first antenna to turn off and the second antenna, locatedat 90° to turn on. From the orthogonal field information, the user cannow locate the receiving antenna on an orthogonal equipotential line.The two-dimensional location of the receiving antenna is revealed by theintersection of the two equipotential lines. Of course it would also bepossible to simply rotate a single transmitting antenna, activating itsequentially in two orientations. The invention of course is not limitedto use in orthogonal coordinates. For many applications, however,orthogonal coordinates provide required accuracy with the greatest speedand resolution.

The novel antenna apparatus comprises a voltage divider to which iscoupled to conducting finger elements. Each finger element has theelectric potential of the voltage divider at the point where they areelectrically coupled. If a radio frequency signal is applied to thevoltage divider, the finger elements will radiate a field that isconstant (at any given point in time) along the fingers but which has agradient across the fingers. The gradient is a reflection of thegradient along the voltage divider. Thus if the finger elements areparallel and straight, a series of equipotential lines parallel to, say,a Y coordinate can be generated. Orienting this antenna at 90°, or usinga second similar antenna oriented at 90°, will provide a set ofequipotential lines parallel to, say, the X coordinate. Each antenna, sodesigned, will locate a receiving antenna in one dimension. Theinvention is not limited to rectilinear coordinate systems. It worksequally well in spherical or other coordinate systems.

In one embodiment of a two dimensional location device, the antennaapparatus of the present invention comprises a first and second antennaseparated by an insulator, the first antenna including a plurality offirst conductive fingers coupled to a plurality of voltage taps of afirst voltage divider, wherein the first conductive fingers are spacedapart from each other and the voltage of each of the first conductivefingers is a preselected fraction of the total voltage applied between afirst input voltage contact and a second input voltage contact of thefirst voltage divider; the second antenna including a plurality ofsecond conductive fingers coupled to a plurality of second voltage tapsof a second voltage divider, wherein the voltage of each second set ofconductive fingers is a preselected fraction of the total voltageapplied between a first input voltage contact and a second input voltagecontact of the second voltage divider; wherein the first and the secondantenna are electrically isolated from each other and wherein thefingers of the first and the second antenna are non-parallel in adetection region of the substrate; whereby the first antenna and thesecond antenna may be used to generate intersecting equipotential linesin the detection region. In a preferred embodiment the voltage divideris a resistive strip having the transmitting fingers coupled along theedge of the resistive strip.

One object of the present invention is to provide an antenna apparatusfor electrographic system with improved control of the equipotentialcontours. A specific object is to reduce curvature, improve linearityand improve orthogonality of the equipotential contours. In a preferredembodiment, the fingers of the first antenna are oriented in localizedregions in a direction generally orthogonal to the fingers of the secondantenna.

Another object of the present invention is to provide an antennaapparatus in which the voltage of each finger may be calibrated. In apreferred embodiment additional calibration elements are included topermit the relative resistance of each segment of the voltage divider tobe adjusted to compensate for processing variations in the resistivityand thickness of the resistors.

Still another object of the present invention is to provide a method offabricating the antenna apparatus as a planar unit and then molding theantenna into a three-dimensional shape. In one embodiment, the antennaapparatus is formed as a planar unit and then molded into ahemispherical shape.

Still yet another object of the present invention is the use of theantenna apparatus as part of an electrographic position detectionsystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art drawing of an electrographic position sensingsystem for determining the position of a stylus.

FIG. 2 is a prior art drawing illustrating the method to determine theposition of the stylus of FIG. 1.

FIG. 3 is a prior art block diagram of the system of FIG. 1.

FIG. 4 is a prior art block diagram of an electrographic positionsensing system for use in determining the position of a stylus relativeto the surface of a globe.

FIG. 5A shows illustrative equipotential lines for a prior art resistivelayer with two contacts energized.

FIG. 5B is a prior art diagram showing superimposed equipotential linesfor a resistive layer similar to FIG. 5A with alternate sets of contactsenergized.

FIG. 6A is a first embodiment of the apparatus of the antenna apparatusof the present invention having impedance elements arranged as a voltagedivider to control the voltage of radiative fingers.

FIG. 6B shows a variation of the antenna apparatus of FIG. 6A in whichthe impedance voltage divider comprises a continuous strip of aresistive material.

FIG. 7 is an illustrative plot of the superimposed equipotential linesof the antenna apparatus of FIG. 6A.

FIG. 8 shows a variation of the antenna apparatus of FIG. 6A withadditional calibration elements coupled to the voltage divider tocalibrate the voltage of each radiative finger.

FIG. 9 shows a variation of the antenna apparatus of FIG. 6A in whichthe finger elements are curved over a portion of the antenna apparatus.

FIG. 10A shows an embodiment of the antenna apparatus of the presentinvention formed from a moldable substrate.

FIG. 10B shows the antenna apparatus of FIG. 10A molded into a coneshape.

FIG. 11 shows a moldable antenna of the present invention molded intothe profile of a child's toy.

FIG. 12 is a rear view of a preferred embodiment of a planar antennaapparatus.

FIG. 13 is a front view of the antenna system of FIG. 12.

FIGS. 14A and 14B show a detail of a preferred embodiment of the antennaapparatus of FIG. 12 in which the two antennas have finger elementsshaped to reduce the interaction between the two antennas.

FIG. 15A shows an illustrative front view of an antenna apparatus shapedto be molded into a hemispherical shape.

FIG. 15B shows an illustrative rear view of the antenna apparatus ofFIG. 15A.

FIG. 15C shows a preferred embodiment of an antenna apparatus shaped tobe molded into a hemispherical shape with the two antennas shownsuperimposed.

FIG. 16 is a block diagram of an electrographic position sensing systemof the present invention including a planar antenna apparatus.

FIG. 17 is a block diagram of an embodiment of the present invention inwhich the voltage divider has center voltage input taps.

FIG. 18 is a block diagram of an electrographic position sensing systemof the present invention for use in a document interpreting system.

FIG. 19 is a block diagram of an electrographic position sensing systemfor use with a globe.

FIG. 20 is a schematic exploded diagram of the position sensing systemof FIG. 19 in use.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally comprises an electrographic positionsensing system, including antenna apparatus for an electrographicposition location system, a method of fabricating the antenna apparatus,and the use of the antenna apparatus in an electrographic positionlocation system. As used in this application, an electrographic locationdetection system is a system in which an antenna system generates aradiating electric field which may be detected a short distanceproximate to the antenna surface by a stylus containing a receivingantenna element. Consequently, as used in this application, anelectrographic location position system may detect the position of astylus brought within a preselected distance from the active surface.

For detection systems similar to those shown in FIGS. 1-4 in which theequipotential lines are generated by radio frequency (rf) drive signals,for example a frequency of about 60 kHz, the equipotential lines willhave an instantaneous potential that is time-varying with the same timedependency as the rf input signal. The range of frequencies that can beused varies widely, with no theoretical limit. The practical limits tothe range of frequencies that can be used depend on the constraints ofany particular application of the technology. The electric fieldsgenerated by the time changing potentials may be calculated usingwell-known laws of electromagnetism, i.e., the electric field is thegradient of the voltage potential. Consequently, maps of theequipotential lines may be calculated and used to interpret the detectedsignal strength proximate the radiating antenna system associated withan active surface.

FIGS. 5A and 5B show the complex set of equipotential lines generated bythe prior art, solid resistive sheet. FIG. 5A is a top view illustratingthe equipotential signal lines for a conventional resistive surface 500with corner contacts 502, 504, 506, and 508. For the purposes ofillustration, it is assumed that sheet 500 has a constant resistivitythroughout the entire surface. Mathematical equations may be used tocalculate the equipotential lines when a particular set of contacts isenergized. Because the current is injected from corner contacts theequipotential lines assume a non-linear shape throughout most of sheet500. Generally speaking, current injected from a particular set ofcontacts will spread throughout the sheet and the potential at any givenpoint may be calculated using well-known equations. When two contacts,such as contacts 502 and 504 are driven by an rf source, resistivesurface 500 radiates a signal above the surface of surface 500. Thecontours of the field lines 500 can be calculated from mathematicalequations. Generally speaking a contour line 507 close to the center ofsurface 500 will be comparatively linear. Contour lines disposed furtheraway from the center, such as lines 508 and 510, will be significantlynon-linear. Substantial non-linearity of the equipotential contour linesoccurs close to the corners, as indicated by lines 510, 512, 514, 516,520, 522, 524, 528, 530, 532, 534, and 536. Note that an orthogonal pairof contacts, such as contact 506 and 504 may also be driven. Byalternating the sequence of contacts which are activated, bothhorizontal and vertical equipotential lines may be generated. FIG. 5B isa prior art plot of the two sets of super-imposed equipotential linesfor the case that contacts 502-504 and 504-506 are sequentially pulsedas part of a multi-state drive sequence. It can be seen that in thecenter of resistive layer 500 the super-imposed equipotential linesapproximate a grid-like pattern with orthogonal cells. Because theequipotential lines in the center of surface 500 are approximatelyorthogonal but become approximately parallel toward corners, theposition determination is worse at the corners than at the center.Additionally, the complexity of calculation is high because of theequipotential lines have complex curves and are non-linear in theirdistribution. This leads to costly and slow electronics.

Additionally solid two-dimensional surfaces that depend on uniformresistance, must have their variations in resistance compensated withcomplex two-dimensional algorithms. This results in costly and slowelectronics.

The novel antenna apparatus comprises a voltage divider to which iscoupled to conducting finger elements. Each finger element has theelectric potential of the voltage divider at the point where they areelectrically coupled. If a radio frequency signal is applied to thevoltage divider, the finger elements will radiate a field that isconstant (at any given point in time) along the fingers but which has agradient across the fingers. The gradient is a reflection of thegradient along the voltage divider. Thus if the finger elements areparallel and straight, a series of equipotential lines parallel to, say,a Y coordinate can be generated. Orienting this antenna at 90°, or usinga second similar antenna oriented at 90°, will provide a set ofequipotential lines parallel to, say, the X coordinate. Because thenovel antenna has open spaces between the finger elements, two antennascan be conveniently stacked. When the top antenna is turned off, theantenna on the bottom can radiate upwards through the open areas in thetop antenna. To prevent electrical shorting, an insulator is placedbetween the antennas.

Each inventive transmitting antenna will locate, in one dimension, areceiving antenna that is placed over it. That is, if the receivingantenna senses a signal of a particular strength, it can be located overor between the finger elements transmitting at that potential.

The area over the transmitting antenna where a receiving antenna can besensed can be termed the “active area”. Since one transmitting antennalocates the receiving antenna in one dimension, the there is a “1-Dactive area” over the finger elements of one transmitting antenna, and a“2-D active area” over the area where two antennas are stacked, that isthe space above stacked antennas into which both transmitting antennasradiate. If two antennas partially overlap, they may have some regionsover them that are 1-D active areas and some regions that are 2-D activeareas.

The invention is not limited to rectilinear coordinate systems. It worksequally well in spherical or other coordinate systems. It is also notlimited to one and two dimensional sensing.

FIG. 6A is a top view of a preferred embodiment of the active surface ofthe present invention comprising two antennas for generating twodifferent sets of equipotential lines. An electrically insulatingsubstrate 670, such as a layer of insulating paper, plastic, fiberglassor Mylar, is used as a support structure. In a preferred embodimentsubstrate 670 has a uniform thickness, t. On a first side 672 ofsubstrate 670 is disposed a first antenna 680. A second antenna 690,shown in phantom, is spaced apart from first antenna 690 and separatedby a sufficient thickness of insulating material to electrically isolateit from first antenna 680. In a preferred embodiment, second antenna 690is disposed on a second side 674 of substrate 670. The two antennas areconfigured so that each antenna can be driven independently of the otherantenna. First antenna 680 has drive terminals 692 and 694 for applyingan rf voltage to first antenna 680. Second antenna 690 has driveterminals 696 and 698 for applying an rf voltage to second antenna 690.

For the purposes of illustrating the principles of the presentinvention, a small number of radiative finger elements are shown,although it will be understood that an arbitrary number of fingerelements may be used. Referring to first antenna 680, an impedancevoltage divider is formed by the impedances of elements 602, 604, 606,608, 610, 612, 614, 616, and 618. The voltage between each impedanceelement is a fraction of the total rf drive voltage between terminals692 and 694, and may be calculated using well known voltage dividerrules. First antenna 680 has low resistance, conductive, finger elements630, 632, 634, 636, 638, 640, 642, and 644. Element 630 is coupled to anode between impedance elements 602 and 604; element 632 is coupled to anode between impedance elements 604 and 606; element 634 is coupled to anode between impedance element 606 and 608; element 636 is coupled to anode between impudence elements 608 and 610; element 638 is coupled to anode between impedance elements 610 and 612; element 640 is coupled to anode between impedance elements 612 and 614; element 642 is coupled to anode between impedance elements 614 and 616, and element 644 is coupledto a node between impedance elements 616 and 618. The voltage of eachfinger element is uniquely determined by the location at which itcouples to the voltage divider. Consequently, each conductive fingerelement radiates a field having magnitude that is a function of thevoltage at the node where the element couples to the voltage divider. Asshown in FIG. 6A, finger 630 has a voltage V1, finger 632 a voltage V2,finger 634 a voltage V3, finger 636 a voltage V4, finger 638 a voltageV5, finger 640 a voltage V6, finger 642 a voltage V7, and finger 644 avoltage V8. Note that in the preferred embodiment that the fingers aresubstantially parallel over a localized region. Consequently, thepotential between finger elements will tend to vary approximatelylinearly with distance between two fingers. The assembly of fingerelements thus radiate a spatially continuous set of equipotential lines,the radiated equipotential lines being substantially parallel to thefinger elements. The term equipotential line relates to a line having aconstant signal amplitude along the line. Other constant magnitudeparameters, such as phase, may be used to achieve the same results.Examples herein are based on signal amplitude but the invention is notso limited.

Second antenna 690, which is disposed on the opposed side of substrate670, has a second voltage divider comprised of second set of impedanceelements 650, 652, 654, 656, 30 and 658. The voltage at the node betweeneach impedance element may be calculated to be a fraction of the totalvoltage applied between terminals 696 and 698 using well known voltagedivider rules. Conductive finger elements 662, 664, 666, and 668 areeach coupled to a respective node between the second set of impedanceelements, thereby establishing a voltage on each finger element. Asshown in FIG. 6A, finger element 662 is an equipotential surface havinga voltage V1′, finger element 664 is an equipotential surface having avoltage V2′, finger element 666 is an equipotential surface having avoltage V3′, and finger element 668 is an equipotential surface having avoltage V4′.

Importantly, because this is an rf system with each finger elementacting as an rf antenna, the broadcast through the air creates acontinuous or smooth rf field resulting in higher resolution in fieldstrength measurement than would be possible by measuring the discreetvoltages generated by each element. The continuous set of field strengthvalues between the elements enables high resolution positionmeasurements to be made. Additionally, each finger element transmits afield that is directly related to the voltage at the point where thefinger couples to the voltage divider. Thus the net effect of all thefingers creates a single field which field potential distributionreflects the voltage distribution along the voltage divider.

Therefore, the voltage divider design, that is, the voltage drop alongthe voltage divider, controls the potential distribution of thetwo-dimensional field broadcast by the antenna finger elements. Thestructure of voltage divider and associated finger element antennas canbe used to create a two-dimensional potential field distribution that islinear and straight.

A line of position in the two-dimensional field can be determined bymeasuring the potential and because the potential distribution is easilymade linear and straight the required electronics and algorithms aresimplified.

By using two such antennas, each comprised of a voltage divider andfinger elements, it is possible to create two separately generatedfields. If the antennas are positioned relative to one another suchthat, (i) their finger element areas overlap, (ii) the finger elementsof one antenna are orthogonal to the finger elements of the otherantenna, and (iii) the finger elements of the top antenna do notsignificantly block signals from the finger elements of the bottomantenna, then two orthogonal, two-dimensional, potential distributions,each of which is linear and straight, can be generated.

Therefore a point of position in the two two-dimensional fields can bedetermined by measuring the potential detected from each antenna. Ofcourse, the two antennas and the fields they broadcast need not belinear, straight, or orthogonal. Having them so simplifies theelectronics and algorithms.

It is an important benefit of this design that each of the twotwo-dimensional potential field distributions can be controlledindependently by adjusting the one dimensional voltage distributionalong the voltage divider associated with each antenna. The antennasystem has an important benefit because variation in resistivity thatoccurs during manufacture of the voltage divider can be compensatedalgorithmically, by using two independent one-dimensional algorithms.This significantly simplifies the electronics and algorithms compared tothe two-dimensional approach the prior art requires.

While the voltage dividers may be fabricated as a series of discreteresistors coupled end-to-end, a preferred method of fabrication is tofabricate the voltage divider as a resistive strip 601, as shown in FIG.6B. The resistive strip may be fabricated with a constant resistance perunit length. Alternately, the resistive strip 601 may have a variableresistance per unit length. The relative voltage at a particular nodewhere a finger element, such as element 632, is coupled to resistivestrip 601, may be calculated using well known voltage divider equations.

As shown in FIG. 7, the finger elements of each antenna 680, 690 formwell-defined equipotential lines. Consequently, the equipotential linesof the two antennas can be designed to be substantially orthogonal toeach other as well as linear and straight. This facilitates positiondetection. The two antennas 680 and 690 of antenna system 600 arepreferably sequentially pulsed at different times according to amulti-state drive method described below in more detail. As shown inFIG. 7, a point, P, may have several unique potentials associated witheach of several unique drive sequences. For example: the potential withboth antennas off, the potential with both antennas on, the potentialwith only the first antenna on, and the potential with only the secondantenna on.

For a pair of antennas wherein each has a resistive strip type voltagedivider, and the voltage divider has finger elements coupled to itbetween at least two electrical contacts, a Five State Drive Algorithmis preferably used to determine the position of a detector, i.e. areceiving antenna, over the pair of transmitting, or broadcasting,antennas. The algorithm is comprised of sequencing through five states,then manipulating the measurements made at each state to obtain thelocation of the device that detected the field strength measurements.Typically that device is a stylus that contains an rf receiving antenna.In many embodiments, a stylus is used to point to a region overlying thetransmitting antenna pair and the receiving antenna in the stylusdetects the magnitude of the electric field strength. The detectedsignals are transmitted to a microprocessor. The five states that aremeasured by the receiving antenna are:

1. no voltage is applied to either antenna;

2. a gradient voltage is applied to the voltage divider of only the topantenna,

3. a constant voltage is applied to the voltage divider of only the topantenna;

4. a gradient voltage is applied to the voltage divider of only thebottom antenna; and

5. a constant voltage is applied to the voltage divider of only thebottom antenna.

Following this sequence, first the potential measured by the stylusduring state I is subtracted from each of the other four measurements toremove any DC error component. After the subtraction, there are fourmeasured field potential values: P_(Top-G); P_(Top-C); P_(Bottom-G); andP_(Bottom-C), respectively, where “G” refers to application of agradiant voltage to the voltage divider and “C” refers to application ofa constant voltage to the voltage divider. Second, to remove anyvariation attributable to the receiving antenna possibly being atdifferent heights with respect to the underlying broadcasting antennapair, each gradient measurement is normalized to the constant voltagemeasurement for both the top and bottom antenna. Thus for the topantenna a value is obtained for the ratio P_(Top-G)/P_(Top-C)=P_(Top)and for the bottom antenna a value is obtained for the ratioP_(Bottom-G)/gP_(Bottom-C)=P_(Bottom). When these ratios are obtained,each is compensated, if necessary, for any variance in resistance alongthe voltage divider (as explained elsewhere, this compensation may havebeen already made by physical devices inserted in the circuitry). Last,the positional meaning of each of the two values, P_(Top) and P_(Bottom)is determined in terms of physical co-ordinates through use of analgorithm based on the designed equipotential line distribution.

Many different antenna patterns can be used to determine the position ofa detector, i.e. a receiving antenna, over the pair of transmitting, orbroadcasting, antennas. In one alternate antenna configuration, the topand bottom antennas may have different geometric configurations. In onesuch embodiment, a first (optionally the “Top”) antenna has a resistivestrip voltage divider coupled to curved finger elements that looparound, say, a hemispherical surface, as shown in FIG. 15A. The loops1510 can have a single break in them, but preferably encircle a“latitude”. Specifically, loops 1510 are substantially parallel to theline of the second antenna's voltage divider 1560, shown in FIG. 15B.The second (optionally the “Bottom”) antenna has a continuous resistivestrip voltage divider 1560 which has no terminal ends, for example,forming a circle. The voltage divider 1560 has at least 3 electricalcontacts placed at intervals along the continuous divider. The contactscan be driven with multiple drive sequences to form specific continuousfields. The bottom antenna also comprises a plurality of fingerelements. They may be approximately evenly distributed throughout thedetection area. This antenna pair is typically used on a hemisphericalshape. Though it is not so limited, the example below will address thehemispherical example.

To determine the position of a receiving antenna located above thehemisphere a Six State Drive Algorithm is used.

1. no voltage is applied to either antenna;

2. a gradient voltage is applied only to the voltage divider 1520 of thetop antenna;

3. a constant voltage is applied only to the voltage divider 1520 of thetop antenna;

4. a first gradient voltage is applied only to voltage divider 1560 ofthe bottom antenna, by applying a voltage to two or more of the at leastthree contacts;

5. a second gradient voltage is applied only to voltage divider 1560 ofthe bottom antenna, by applying a different voltage pattern to two ormore of the at least three contacts; and

6. a constant voltage is applied only to the voltage divider of thebottom antenna. Following this sequence, first the potential measured bythe receiving antenna in a stylus during state 1 is subtracted from eachof the other five measurements to remove any DC error component. Afterthe subtraction, there are five measured field potential values:P_(Top-G); P_(Top-C); P_(Bottom-G1); P_(Bottom-G2); and P_(Bottom-C),respectively.

Second, to remove any variation attributable to the receiving antennapossibly being at different heights with respect to the underlyingbroadcasting antenna pair, each gradient measurement is normalized tothe constant voltage measurement for both the top and bottom antenna.Thus for the top antenna a value is obtained for the ratioP_(Top-G)/P_(Top-C)=P_(Top) and for the bottom antenna a value isobtained for the two ratios P_(Bottom-G1)/P_(Bottom-C.)=P_(Bottom1), andP_(Bottom-G2)/P_(Bottom-C.)=P_(Bottm2). When these three ratios areobtained, each is compensated, if necessary, for any variance inresistance along the voltage divider. Last, the positional meaning ofthe values of P_(Top), P_(Bottom1), and P_(Bottom2) are determinedthrough use of an algorithm based on the designed equipotential linedistribution. Two values are needed to uniquely determine theco-ordinate value of the second co-ordinate, associated with the bottomantenna, because the potential that is measured could be at either oftwo points on the equipotential line generated from a particulargradient drive pattern. Thus points need to be measured on equipotentiallines generated from two drive configurations to obtain a uniqueco-ordinate point.

The antenna designs of the present invention also permit the relativeequipotential of each finger element to be adjusted. FIG. 8 shows anembodiment 800 of the present invention designed to permit additionalcontrol of the voltage of each element 830, 832, 834. As shown in FIG.8, a voltage divider comprised of resistors 810, 812, 814, and 816 iscoupled end-to-end. The two ends of the chain, nodes 802 and 804, areconfigured to be driven by an rf voltage source. Normal manufacturingvariance may, however, result in each of the resistors 810, 812, 814,and 816 being 20-30% away from their nominal target values. Additionalresistive tuning elements 820, 822, 824, and 826 are configured topermit each resistor to have its effective value trimmed. Resistivetuning elements 820, 822, 824, and 826 may comprise any conventionalelement used to trim a resistance value. For example, resistive tuningelements 820, 822, 824, and 826 may comprise a resistor that is trimmedin physical thickness by mechanical trimming or by laser trimming.Additionally, resistive tuning elements 820, 822, 824, and 826 maycomprise any conventional arrangement of resistors and fuses configuredto permit the effective resistance of resistive tuning elements 820,822, 824, and 826 to be adjusted. To compensate for variation inresistivity along resistive strip 601, holes can be created or punchedin the strip.

Yet another way to compensate for variable resistance along the voltagedivider is to use one or more input voltage taps as illustrated in FIG.17 and described in more detail below.

One of the advantages of the antenna system of the present invention isthat it provides a way to control the equipotential profile of surfaceswith complex shapes. It is preferable that the finger elements of twoantennas 680, 690 define an orthogonal grid because this reduces thecomputational difficulty of calculating a position based upon measuredvoltages at a particular point in space. However, the present inventionmay be adapted for use in a variety of curved and non-planar surfaces aswell. FIG. 9 shows an embodiment of the present invention in which thefingers 910 of a first antenna 920 have a first radius of curvature overa two dimensional surface 905. First antenna 920 has resistive elements930, 932, 934, 936, and 938 coupled together as a voltage divider todetermine the voltage of each finger element 910. Second antenna 940 isshown in phantom and is spaced apart from surface 905 underlying firstantenna 920. Second antenna has finger elements 950 with the voltage ofeach of the elements determined by the node at which the element iscoupled to the voltage divider, between resistors 962, 964, 966, and968. The finger elements 950 of second antenna 940 may have a secondradius of curvature. A point P located in the region ABCD formed betweenthe finger elements of first antenna 920 and second antenna 940 hassignificant symmetry. Consequently, the voltages at point P will be aquasi-linear function of the separation of point P from line segments ABand DC and of the separation of point P from line segments BC and AD.

A preferred method of fabrication is to form the antennas on aninsulating substrate, such as a plastic or Mylar substrate. A preferredmethod to fabricate resistive and conductive elements is with patternedconductive ink films. Any resistive material may be used to fabricatethe resistors of a voltage divider, such as a carbon based polymer ink,or carbon based water ink. The fingers may be formed from a conductiveink or a thin layer of metal. A preferred low cost construction approachis to use high speed printing techniques to print carbon and silver inkson either Mylar or paper substrates. Silkscreen techniques work best forpolymer based inks whereas flexographic and Graveur process work bestfor water based inks.

The resistive and conductive layers may also be patterned on a substratethat can be formed by a vacuum molding process. In particular, thesubstrate may comprise a flat vinyl sheet. This permits the antenna ofthe present invention to be patterned as a two-dimensional planarsurface and then later molded into a more complex three-dimensionalshape using well known vacuum molding processes. FIG. 10 A is a top viewof an antenna system 1000 formed on a moldable substrate 1005. Firstantenna 1010 includes a first set of finger elements 1020 disposed on afirst surface of moldable substrate 1005. Each of the first set offinger elements is coupled to a different node of a first resistordivider 1030, which comprises a resistor with a preselected resistanceper unit length. A second antenna 1040 is disposed on an opposed sideand is shown in phantom. The finger elements of the second antenna areshown as forming loops. Each of the finger elements of second antenna1040 are connected to a different node of a second resistor divider 1060having a preselected resistance per unit length. Antenna system 1000 maybe molded using conventional plastic and polymer molding processes, suchas vacuum or pressure forming molding. For example, as shown in FIG.10B, antenna system 1000 may be molded into a cone shape. A point P onthe planar surface is translated to a point P* on the cone shapedsurface of FIG. 10B. As can be seen in FIG. 10B, in a region around P*the finger elements retain local symmetry. Consequently, theequipotentials will tend to be a quasi-linear function of positionrelative to the grid formed by neighboring finger elements.

When a planar antenna system is formed into a complex shape the positionfinding problem can be broken into two elements for the purpose ofretaining simplified algorithms.

The first element consists of the position finding problem that appliesto the antenna pattern as it exists on the planar substrate prior toforming. The second element is to apply a translation algorithm or mapthat represents the physical transformation of the planar surface intothe complex surface.

The antenna system of the present invention may also be molded into avariety of shapes. As shown in FIG. 11, an antenna system 1110 may bemolded into a complex surface, such as that of a portion 1102 of achild's toy 1104. First antenna 1120 and second antenna 1130 may besubstantially orthogonal over localized surface regions even though thesurface as a whole is highly non-planar. Consequently, the equipotentialabout a local point P of toy 1104 will tend to be quasi-linear functionsof position with respect to neighboring finger elements.

FIG. 12 shows a view, through a transparent insulating sheet, of apreferred embodiment of an antenna apparatus 1200 printed on theopposite side of the sheet from that viewed. In this embodiment theposition of stylus relative to a planar support surface is determined. Aresistive strip 1203, driven by end contacts 1201 and 1202, forms avoltage divider for driving the finger elements of second antenna 1210.FIG. 13 shows a view of the antenna apparatus of FIG. 12 having a secondantenna 1310 printed on the side viewed. A stylus 1304 is shownpositioned over the surface of antenna 1310. Antenna 1310 has fingerelements 1320 driven by a resistive strip 1330, configured as firstvoltage divider between contacts 1340 and 1350.

Several factors are balanced in choosing the optimum distance, or gap,between finger elements. Generally it is desirable, on the top antenna,for the distance between finger elements to be large enough that thebroadcast from the bottom antenna is not blocked by the structure of thetop antenna. In addition, the wider the spacing is between the fingers,the smaller the capacitive coupling will be between the two antennas,which are optimally uncoupled. Based on these factors, it would seemdesirable to design wide spaces between the finger elements. However, asthe distance between the finger elements increases, their ability tobroadcast a signal efficiently decreases. Furthermore, if the distancebetween the finger elements is widened by decreasing the finger width,the antenna resistance increases, which leads to detrimental crosscoupling between the antennas. Thus choosing a distance between thefinger elements involves balancing all of these factors.

Another set of factors that require balancing relate to the width ofeach finger element. Wide fingers tend to produce good broadcastcharacteristics and result in less interference from other objects, likefor example, a user's hand grasping the stylus to point. But wide fingerelements have the disadvantage of increasing capacitive coupling betweenthe antennas. This is because the capacitive coupling between the twoantennas is a direct function of the overlap of the fingers from each ofthe two antennas. If the finger widths are wider, there is more overlaparea. To address this problem, finger elements were designed that werenarrower at the points were fingers from two antennas would cross, andwider at other regions. This is illustrated in FIG. 14B. Fingers 1410are located on the top antenna and, compared to the bottom antenna, haveless-broad widened areas so as not to obstruct the field from the bottomantenna, which must radiate through the top antenna. Fingers 1420 arelocated on the bottom antenna and have relatively broader, thoughshorter, widened areas so that the bottom antenna can radiateeffectively between the fingers of the top antenna. Of course theinvention is not limited by the exact design of the variable widthfinger elements. Rather, the geometry is optimized for each applicationof the novel antennas.

It is not necessary to form the antennas on opposing sides of a singleelectrically insulating sheet. They may be formed on separate sheets,then sandwiched together in a configuration where the fingers of oneantenna are not parallel to the fingers of the other antenna, and wherethe antennas are separated by one of the insulating sheets. Howeverforming the antennas in this fashion has some disadvantages. Onedisadvantage is that the two sheets must be consistently aligned forconsistent and error-free operation. Another is that in non-planarconfigurations it may be difficult to fit the two sheets immediatelyadjacent to one another. Variable spacing between the two antennas couldresult in decreased sensitivity and accuracy.

FIG. 16 is a block diagram of an electrographic position sensing systemincluding an antenna apparatus similar to that shown in FIGS. 12-14. Itis illustrated for use in a planar configuration. In one embodiment aprocessor, preferably a microprocessor, controller 1601 regulates theoperation of the active antenna apparatus 1621 and receives positiondata 1617 which it uses to determine the position of a stylus 1611 nearactive area 1609 proximate to the finger elements of antenna apparatus1621. Controller 1601 also includes a user interface 1618 and an audioblock 1619 for outputting an audio output via a speaker 1620.

As shown in FIG. 16, controller 1601 sends commands 1602 to transmittinglogic block 1603 to cause a sequence of transmitting signals to performa position detection function. The commands 1602 may include beginningand/or stopping position sensing. Additionally the commands 1602 mayalso be in regards to the desired resolution, i.e., commands 1602 mayalso include instruct transmitting block 1603 to adjust the mode ofoperation to achieve a desire resolution or speed for a particularapplication.

Transmitting block 1603 drives the two antennas of antenna apparatus1620 according to predetermined multi-state drive sequence. In apreferred embodiment, two antennas each having a resistive voltagedivider strip are used. The antennas are driven using the Five StateDrive algorithm described above. The drive signals of transmitting logicblock 1603 are preferably amplified with amplifiers 1604 and transmittedvia wires having wire shielding 1605. Each antenna has two electricalcontacts 1606 driving a resistive voltage divider 1607 which is used tosupply the voltages to the fingers 1608 of each antenna.

Stylus 1610 has an conductive element which receives the transmittedsignals. A conductor with a ground shield 1611 conducts the receivedsignals to a receiving amplifier 1612. The receiving amplifier 1612 mayperform any conventional gain, filtering, and DC rejection function toamplify and condition the received signals. The conditioned signals areset to signal detection block 1613 which performs demodulation, analogto digital conversion, and optionally integrated. In a preferredembodiment synchronous demodulation of a single frequency signal is usedbecause this enhances the signal to noise ratio. However, synchronousdemodulation requires timing signals 1615 and 1616 to coordinate theactivities of signal detection block 1613. In a preferred embodiment,signal detection block 1613 integrates the signal to achieve narrow bandfiltering and uses a constant slope discharge technique to convert theintegrated signal to a digital value for interpretation by the receivelogic block 1614. The receive logic block 1614 directs the receivedsignal detection process with receive timing signals 1616. For the casethat synchronous demodulation is used, transmit timing information 1615included with the receive timing signals 1616. The receive logic block1614 accepts digital data from the receive signal detection block 1613and formats the data as appropriate for delivery to controller 1601.

FIG. 17 is a block diagram showing a portion of the system of FIG. 16 inwhich the transmitting logic block 1603 and voltage resistive voltagedivider 1607 are modified for improved position accuracy. As shown inFIG. 17, resistive voltage divider 1607 may include one or moreadditional input voltage taps to more precisely define the voltage alongthe resistive voltage divider 1607.

One application of the antenna apparatus of the present invention was tocreate interactive books. As shown in FIG. 18, the sheets of a booklet1807 were placed over an active surface having at least one antennaapparatus 1803 similar to the antenna apparatus 1609 of FIG. 16. Astylus 1804 was pointed at a portion of an open page of booklet 1807 toidentify a word, letter, or picture. Microprocessor 1801 then calculatedthe position of stylus 1804 relative to antenna apparatus 1803. In apreferred embodiment, a speaker 1806 was used to provide an audio outputas a function of the portion of booklet 1807 to which the user pointedstylus 1804. For comparable electronic accuracy, the resolution of theantenna apparatus 1621 of the present invention was about a factor ofthree higher than for previously constructed electrographic sensors inwhich the equipotential lines were generated within the body of aresistive surface. Further, the resolution was maintained at the edgesof the antenna elements, unlike the solid resistive body which had up tofour times less resolution near the edges.

In a preferred embodiment, the antenna system of the present inventionis used to detect the position of a stylus over a platform. The dualtransmitting rf antennas are located in the platform. A receivingantenna, or detector, is located in the stylus. This interactive printmedia or platform system is the subject of a patent application filed bythe assignee of the present application. The co-pending application,incorporated herein by reference, is entitled “Interactive Platform andLocator System,” Ser. No. 60/200,725, filed Apr. 27, 2000.

One application of the antenna apparatus of the present invention is ina globe similar to that shown in FIG. 20. As shown in the rear view ofFIG. 15A, one side of an planar insulative substrate 1505 is patternedwith fingers 1510 shaped as concentric conductive rings. Each of therings is coupled to a different portion of a resistive strip, with theresistive strip acting as a voltage divider dividing the voltage betweencontacts 1530, 1580. Consequently, each concentric ring will broadcast adifferent equipotential line. The opposed side of the insulatingsubstrate is patterned with radially directed finger elements 1550. Eachfinger is coupled to a different node along a resistive voltage divider1560 located at the circumference of the insulating substrate. Thefingers 1550 may also be shaped with a non-uniform thickness in order toimprove the electromagnetic characteristics of the antenna structure.The radial length of each finger may also be varied to alternate longand short radial fingers 1550 so that the azimuthal separation betweenfinger elements is made more uniform. Some of the details and structureare omitted in FIGS. 15A and 15B for the purposes of illustration. FIG.15C shows a preferred embodiment with the shape of the opposed antennassuperimposed. The antenna apparatus is preferably fabricated on a vinylsubstrate using an ink process to fabricate the resistive elements andthe conductive fingers. The two opposed antennas define equipotentiallines which are orthogonal to each other which are a simple function ofradial (r, Θ) coordinates. The fabricated apparatus of FIG. 15C is thenvacuum molded into a hemispherical shape.

FIG. 19 is a schematic block diagram of a position sensing system with ahemispherical antenna system having a first antenna with radial fingerelements 1908 coupled to a circumferential resistive element 1907 and asecond antenna comprised of circular-shaped finger elements 1909 coupledto a radially, or longitudinally, oriented voltage divider 1910.Transmitting block 1903 and amplifiers 1904 are arranged to provide thedrive signals to the antennas according to the Six State Drive Algorithmdescribed above.

FIG. 20 shows an exploded perspective view of a globe having an antennaapparatus shaped as two hemispheres 2001, a plastic disk 2002 whichsupports a transmitting logic block 2003. Electrical contact wires 2008couple transmitting logic block 2003 to electrical clips 2009.Transmitting logic block 2003 is electrically coupled to a support stem2004 to provide a connection to main electronics unit 2006 containing amicroprocessor controller (not shown in FIG. 20). A stylus 2007, whichhas a receiving antenna for receiving signals, is coupled to mainelectronics unit 2006. The globe is preferably supported by a base 2005.

Although a preferred embodiment of the present invention andmodifications thereof have been described in detail herein, it is to beunderstood that this invention is not limited to those preciseembodiments and modifications, and that other modifications andvariations may be affected by one of ordinary skill in the art withoutdeparting from the spirit and scope of the invention as defined in theappended claims.

I claim:
 1. A method for locating a user selected position over anantenna apparatus comprising the steps of: a) providing a firsttransmitting antenna, the first antenna comprising a first voltagedivider having at least two electrical contacts coupled to it, and aplurality of spaced apart, substantially parallel, electricallyconductive, finger elements coupled to the first voltage divider betweenthe at least two electrical contacts; b) providing an electricalinsulator to separate the first transmitting antenna from a secondtransmitting antenna; c) providing the second transmitting antennacomprising a second voltage divider having at least two electricalcontacts coupled to it, and a plurality of spaced apart, substantiallyparallel, electrically conductive, finger elements coupled to the secondvoltage divider between the at least two electrical contacts, the secondtransmitting antenna oriented so that the area defined by its fingerelements overlay a portion of the area defined by the finger elements ofthe second antenna, and the finger elements of the first antenna form anon-zero angle with the finger elements of the second antenna; d)providing a processor coupled to a user interface and further coupledthrough other electronics to the first voltage divider at two or moreelectrical contacts and coupled to the second voltage divider at two ormore electrical contacts; e) providing a drive signal transmittercoupled between the processor and through amplifiers to the firstvoltage divider at two or more electrical contacts and throughamplifiers to the second voltage divider at two or more electricalcontacts, the transmitter capable of receiving commands from theprocessor and transmitting signals to the first and second voltagedividers independently; f) providing a receiving antenna coupled to anamplifier, the amplifier coupled to the processor; g) providing a signaldetector coupled between the receiving antenna amplifier and theprocessor; h) providing a signal receiver coupled between the signaldetector and the processor, the signal receiver further coupled to thedrive signal transmitter; i) placing the receiving antenna at a positionover the area where the finger elements of the first and second antennaoverlap; j) causing the processor to send commands to the drive signaltransmitter, the commands causing the transmitter to send a sequence offive drive-signal states to the first and second voltage dividersindependently, the five states being: i) applying zero voltage to thefirst and the second voltage dividers; ii) applying a gradient voltageto the voltage divider of the first, top, antenna and zero voltage tothe second, bottom, antenna; iii) applying a constant voltage to thevoltage divider of the first, top, antenna and zero voltage to thesecond, bottom, antenna; iv) applying a gradient voltage to the voltagedivider of the second, bottom, antenna and zero voltage to the first,top, antenna; and v) applying a constant voltage to the voltage dividerof the second, bottom, antenna and zero voltage to the first, top,antenna; k) receiving a signal measurement from the receiving antennaduring each drive state; l) detecting a magnitude of the measured signaldata from the receiving antenna and sending to the signal receiver; m)synchronizing the received signal data with timing data obtained fromthe drive signal transmitter; and n) calculating the position of thereceiving antenna from the measured signal data.
 2. The method of claim1 wherein the position of the receiving antenna is calculated accordingthe following steps: a) subtracting the signal magnitude data measuredat state (i) from the signal magnitude measured at each of the fourother states, to yield: P_(Top-G)=the signal magnitude measure at state(ii) less the signal magnitude data measured at state (i); P_(Top-C)=thesignal magnitude measure at state (iii) less the signal magnitude datameasured at state (i); P_(Bottom-G)=the signal magnitude measure atstate (iv) less the signal magnitude data measured at state (i);P_(Bottom-C)=the signal magnitude measure at state (v) less the signalmagnitude data measured at state (i); b) obtaining a single data pointrelating to the measurements taken from the top antenna, P_(Top), bycalculating the ratio of P_(Top-G)/P_(Top-C); c) obtaining a single datapoint relating to the measurements taken from the bottom antenna,P_(Bottom), by calculating the ratio of P_(Bottom-G)/P_(Bottom-C); andd) determining the positional meaning of P_(Top) and P_(Bottom) based ona model of a field radiated by the transmitting antennas.
 3. The methodof claim 3 wherein the position of the receiving antenna is calculatedby the additional step of compensating for variance in resistance ineach voltage divider after P_(Top) and P_(Bottom) are calculated.
 4. Themethod of claim 3 further comprising the step of compensating fornonlinear variation along any voltage divider by providing an algorithmin the processor, the algorithm containing correction parameters for aone-dimensional voltage divider.
 5. A method for locating a userselected position over an antenna apparatus wherein one antenna has aloop voltage divider, comprising the steps of: a) providing a firsttransmitting antenna, the first antenna comprising a first voltagedivider having at least two electrical contacts coupled to it, thefirst, and a plurality of spaced apart, electrically conductive, fingerelements coupled to the first voltage divider between the at least twoelectrical contacts; b) providing an electrical insulator to insulatethe components of the first transmitting antenna from a secondtransmitting antenna; c) providing the second transmitting antenna, thesecond antenna comprising a second voltage divider shaped in a loop andhaving at least three electrical contacts at intervals along the loop,and a plurality of spaced apart, low resistance, finger elements coupledto the second voltage divider at intervals between each two of the atleast three contacts, such that the electrical potential along a eachelement is substantially uniform and the elements are oriented at asubstantially a constant angle with a tangent of the loop where eachelement couples to the loop; d) orienting the finger elements of thefirst antenna to define the area enclosed by the loop of the secondvoltage divider, and orienting the fingers of the second antenna to liewithin the loop of the second voltage divider, wherein the first antennais oriented so that the area defined by the finger elements of the firstantenna overlay a portion of the area defined by the finger elements ofthe second antenna; and the finger elements of the first antenna form anon-zero angle with the finger elements of the second antenna; e)providing a processor coupled to a user interface and further coupledthrough other electronics to the first voltage divider at two or moreelectrical contacts and coupled to the second voltage divider at threeor more electrical contacts; f) providing a drive signal transmittercoupled to the processor and through amplifiers to the first voltagedivider at two or more electrical contacts and coupled throughamplifiers to the second voltage divider at three or more electricalcontacts, the transmitter capable of receiving commands from theprocessor and transmitting signals to the first and second voltagedividers independently; g) providing a receiving antenna coupled to anamplifier, the amplifier coupled to the processor; h) providing a signaldetector coupled between the receiving antenna amplifier and theprocessor; i) providing a signal receiver coupled between the signaldetector and the processor, the signal receiver further coupled to thedrive signal transmitter; j) placing the receiving antenna at a positionover the area where the finger elements of the first and second antennaoverlap; k) causing the processor to send commands to the drive signaltransmitter, the commands causing the transmitter to send a sequence ofsix drive-signal states to the first and second voltage dividersindependently, the six states being: i) applying zero voltage to thefirst and the second voltage dividers; ii) applying a gradient voltageto the voltage divider of the first, top, antenna and zero voltage tothe second, bottom, antenna; iii) applying a constant voltage to thevoltage divider of the first, top, antenna and zero voltage to thesecond, bottom, antenna; iv) applying a first gradient voltage to two ormore of the at least three contacts of the voltage divider of thesecond, bottom, antenna and applying zero voltage to the first, top,antenna; v) applying a second gradient voltage to two or more of the atleast three contacts of the voltage divider of the second, bottom,antenna and applying zero voltage to the first, top, antenna; vi)applying a constant voltage to the voltage divider of the second,bottom, antenna and zero voltage to the first, top, antenna; l)receiving a signal measurement from the receiving antenna during eachdrive state; m) detecting a magnitude of the measured signal data fromthe receiving antenna and sending to the signal receiver; n)synchronizing the received signal data with timing data obtained fromthe drive signal transmitter; and o) calculating the position of thereceiving antenna from the measured signal data.
 6. The method of claim5 wherein the position of the receiving antenna is calculated accordingthe following steps: a) subtracting the signal magnitude data measuredat state (i) from the signal magnitude measured at each of the fiveother states, to yield: P_(Top-G)=the signal magnitude measure at state(ii) less the signal magnitude data measured at state (i); P_(Top-C)=thesignal magnitude measure at state (iii) less the signal magnitude datameasured at state (i); P_(Bottom-G1)=the signal magnitude measure atstate (iv) less the signal magnitude data measured at state (i);P_(Bottom-G2)=the signal magnitude measure at state (v) less the signalmagnitude data measured at state (i); P_(Bottom-C)=the signal magnitudemeasure at state (vi) less the signal magnitude data measured at state(i); b) obtaining a single data point relating to the measurements takenfrom the top antenna, P_(Top), by calculating the ratio of P_(Top-G)/P_(Top-C); c) obtaining two data points relating to the measurementstaken from the bottom antenna, P_(Bottom-G1) and P_(Bottom-G2) bycalculating the ratios of P_(Bottom-G1)/P_(Bottom-C) andP_(Bottom-G2)/P_(P) _(Bottom-C) respectively; and d) determining thepositional meaning of P_(Top), P_(Bottom-G2), and P_(Bottom-G1), basedon a model of a field radiated by the transmitting antennas.
 7. Themethod of claim 6 wherein the position of the receiving antenna iscalculated by the additional step of compensating for variance inresistance in each voltage divider after P_(Top) and P_(Bottom) arecalculated.
 8. The method of claim 6 further comprising the step ofcompensating for nonlinear variation along any voltage divider byproviding an algorithm in the processor, the algorithm containingcorrection parameters for a one-dimensional voltage divider.
 9. Themethod of claim 2 further comprising the step of shaping the first andsecond transmitting antennas and insulator substantially into ahemisphere and orienting the finger elements of the first antenna tosubstantially encircle the hemisphere along a latitude and orienting thefingers of the second antenna to substantially lie along longitudes ofthe hemisphere.